Nutrients and the importance of upwellings in oceans

In the oceans, locally high levels of primary productivity are associated with high nutrient inputs from two sources. First, nutrients may flow continuously into coastal shelf regions from estuar ies. An example is provided in Figure 17.16. Productivity in the inner shelf region is particularly high both because of high nutrient concentrations and because the relatively clear water provides a reasonable depth within which net photosynthesis is positive (the euphotic zone). Closer to land, the water is richer in nutrients but is highly turbid and its productivity is less. The least productive zones are on the outer shelf (and open ocean) where it might be expected that primary productivity would be high because the water is clear and the euphotic zone is deep. Here, however, productivity is low because of the extremely low concentrations of nutrients.

Ocean upwellings are a second source of high nutrient concentrations. ... and upwellings These occur on continental shelves where the wind is consistently parallel to, or at a slight angle to, the coast. As a result, water moves offshore and is replaced by cooler, nutrient-rich water originating from the bottom, where nutrients have been accumulating by sedimentation. Strong upwellings can also occur adjacent to submarine ridges, as well as in areas of very strong currents. Where it reaches the surface, rich supplies of nutrients in marine environments... ... from estuaries...

À A

\ / y \ / y, / \

Phytoplankton productivity

y

Nutrient concentration

Outer shelf

Estuary ' |nner she|f

Outer shelf

20 km

the nutrient-rich water sets off a bloom of phytoplankton production. A chain of heterotrophic organisms takes advantage of the abundant food, and the great fisheries of the world are located in these regions of high productivity.

Recently, iron has been identified as a limiting nutrient that potentially affects about one-third of the open ocean (Geider et al., 2001). Iron, which is very insoluble in seawater, is ultimately derived from wind-blown

Figure 17.16 Variation in phytoplankton net primary productivity, nutrient concentration and euphotic depth on a transect from the coast of Georgia, USA, to the edge of the continental shelf. (After Haines, 1979.)

particulate material, and large areas of ocean receive insufficient amounts. When iron is added experimentally to ocean areas, massive blooms of phytoplankton can result (Coale et al., 1996); such blooms are also likely to occur when large storms supply land-derived iron to the oceans.

While nutrients are the most influential factors for local ocean productivity, temperature and PAR also play a role at a larger scale (Figure 17.17).

Figure 17.17 Relationships between daily depth-integrated estimates of net primary production (NPP) and: (a) sea surface temperature (SST), and (b) above-water daily photosynthetically available radiation (PAR). The different symbols relate to different data sets from various oceans. (After Campbell et al., 2002.)

iron as a limiting factor in oceans temperature and PAR also affect productivity

Figure 17.17 Relationships between daily depth-integrated estimates of net primary production (NPP) and: (a) sea surface temperature (SST), and (b) above-water daily photosynthetically available radiation (PAR). The different symbols relate to different data sets from various oceans. (After Campbell et al., 2002.)

Figure 17.18 (a) The general relationship with depth, in a water body, of gross primary productivity (GPP), respiratory heat loss (R) and net primary productivity (NPP). The compensation point (or depth of the euphotic zone, eu) occurs at the depth (Zeu) where GPP just balances R and NPP is zero. (b) Total NPP increases with nutrient concentration in the water (lake iii > ii > i). Increasing fertility itself is responsible for greater biomasses of phytoplankton and a consequent decrease in the depth of the euphotic zone.

This has significance for our ability to estimate ocean primary productivity because sea surface temperature and PAR (together with surface chlorophyll concentration, another factor correlated with NPP) can be measured using satellite telemetry.

Was this article helpful?

0 0
Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment